With the growing energy crisis and environmental pollution problems, renewable new energy sources have been vigorously developed, and users have put forward higher requirements for distribution networks in terms of distributed power access, load and power demand diversification, power quality, and power supply reliability (Ghareeb et al. 2013; Zhang et al. 2018). As an important approach to Improve the efficiency of distributed power consumption research and investigation of DC microgrids are increasingly available (Brivio at el. 2016; Chatzinikolaou et al. 2017). Due to the random and intermittent nature of distributed power sources, battery energy storage systems with flexible regulation capability are essential in microgrids (Dubarry et al. 2019; Gupta et al. 2018). A lot of research had been done at home and abroad on the topology, connection methods, control of grid-connected and islanding operation methods, and energy management of battery energy storage systems. In (Palizban et al. 2015), a decentralized control method for SOC was proposed, based on a modified droop control method in which the SOC of each battery energy storage units was balanced during the discharge process. A high-efficiency grid-tie lithium-ion-battery-based energy storage system was prorpsed in (Qian et al. 2011), which adopted a highly efficient opposed-current half-bridge-type inverter along with an admittance-compensated quasi-proportional resonant controller to ensure high power quality and precision power flow control. Branco et al. (2018) assessed the possibility of installing battery storage systems into the isolated grid for renewable energy sources integration and the results suggested that the technology considerably decreased the levels of renewable energy sources curtailment. But when an inter-pole short-circuit fault occurs in DC microgrid, the energy storage battery as a voltage source will provide a great short-circuit current to the fault point, endangering system and equipment safety (Drovtar et al. 2012). These above studies did not take into account this issue.
At this stage of engineering, current-limiting reactors are generally installed to limit the diode current after a short circuit. Zhao et al.(2013) analyzed a short circuit transient process of a distributing network with current limiting reactor derived and an engineering calculation formula of current limiting reactor’s impedance.In (Drovtar et al. 2012), proper application of current limiting reactor to high voltage substation was proposed, based on a comprehensive short circuit analysis of 4 well-known substation bus bar arrangements. However, the presence of a current limiting reactor in an electric circuit will lead to a significant increase in the circuit breaker transient recovery voltage, which will also have a greater impact on switching overvoltage and temporary overvoltage. And current limiting reactors are very easy to saturate at high currents and lose their current limiting effect (Chen et al. 2013).
Over the past few decades, Fault Current Limiters (FCL) have been widely used in engineering as a revolutionary power system device to overcome the problems caused by increased fault current levels (Sushma et al. 2014). The FCL’s can be classified as: Superconducting FCL (SFCL), Solid-State FCL (SSFC), Electromagnetic Dynamic Fault Current Limiter (DFCL), Hybrid FCL. SFCL is a novel electric equipment which has the capability to reduce the fault current level within the first cycle of fault current. A DC SFCL was proposed in (Li et al. 2018), which not only limited the peak DC fault current to meet the requirement of reducing the maximum opening capacity of DC circuit breakers, but also limited the fault current rise rate and achieves coordination between the converter, DC circuit breaker and DC SFCL.But because SFCL need to be equipped with expensive cooling systems, they cannot be widely used in DC microgrids where cost requirements are more stringent. Compared to SFCL, SSFC can also achieve a fast response to fault currents with no special requirements for cooling systems. SSFC that limit short-circuit currents by opening IGBTs were discussed in (Fang et al. 2008). However, the control of fault current by IGBT requires high overcurrent resistance of IGBT, and the feasibility and device safety need to be thoroughly investigated.A DFCL is an electromagnetic FCL which automatically and instantaneously adjusts its own impedance depending upon the magnitude of the fault current. There by maintaining the let through current within a narrow range of values. Rubenbauer et al. (2007) proposed a DFCL based on a six-pulse thyristor rectifier which can switch off the limited fault current very fast by blocking the firing pulses.However, the DFCL structure is too complex for DC microgrid and cannot be used in high-power applications, so it has not been widely used in engineering. Prigmore et al. (2014) proposed a hybrid fault current limiter that controls the magnitude of the fault current by pulse-width modulation. This fault current limiter is small in size, but additional components may increase its footprint.
Scholars at home and abroad have done a lot of research on other fault limiting methods. Baran et al. (2007) proposed a protection scheme using a modern voltage-type converter as a fast-acting current-limiting circuit breaker, which effectively limited the fault current by changing the diode in the converter to a fully controlled device. However, this approach will greatly increase the cost and cannot be widely used in DC microgrids where cost requirements are more stringent. Lu et al.(2018) proposed a virtual impedance-based fault current limiting method, which effectively limited the fault current and did not require additional equipment. However, the current limiting effect still had a gap compared to the fault current limiter. In (Ni et al; 2020), a fault-limiting method based on adaptive control was proposed. This method has a limiting effect on the fault current without adding additional current limiting devices, but the effect is not very obvious, and the control strategy of the converter will become very complicated after the introduction of adaptive control.
The above-mentioned current limiting methods of battery energy storage systems are mostly with the help of current limiting reactors, fault current limiters and other current limiting devices, which are more costly and less effective in promotion. The current limiting effect of the fault current limiting methods without relying on the current limiting device are also not ideal, and they are more difficult to cooperate with the protection. In this paper, a fault current control method for DC microgrid battery storage system is proposed. Unlike the existing current limiting methods, this paper does not add any current limiting device, but increases the rated voltage of the battery stack of the battery storage system, which can be connected to the DC microgrid through a Buck DC/DC converter. In case of inter-pole short-circuit fault, the Buck DC/DC converter can reduce the short-circuit current provided by the battery storage system by buck operation, and the short-circuit current can be controlled transparently. The protection of the DC microgrid is easily achieved by combining with DC circuit breakers.